I am trying to build a chiller but I forget if I have heard that the copper tubing should be 3/8 in. diameter on the inside or outside. Or maybe it's not that measurement at all. Anybody want to comment on how much they have payed to make one of these. Thanks.

I don't if it is OD or ID but I think that when you buy copper tubing you get it in 3/8 inch as any larger and you won't get as efficient heat transfer. I cheated and bought mine. In retrospect I should have made it but it was an impulse buy at the LHBS. I think I paid 40 bucks for the phils chiller.